refactor(settings): one bounded_float for every numeric bar read from settings

The shape ledger's divergence readout flagged _gate_setting (#4385).
Reading it turned up a family with no canon: floor_for,
get_duplicate_threshold, get_plan_match_threshold and _gate_setting each
parsed a stored string, fell back to the default (never 0) and clamped
into [lo, 1].

- services/settings.bounded_float(raw, default, lo=0, hi=1) is the pure
  parse, fallback and clamp. Each caller keeps its own get_setting read,
  so tests patching get_setting per module still take effect, and
  _gate_setting still fails open on an unreadable setting.
- SettingsView.saveKbInject: eleven inline Math.min/Math.max clamps and
  the local gateAt become one asBar(v, d, lo = 0), mirroring the server.

No behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/services/retrieval_surfaces.py

@@ -66,7 +66,7 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 
-from scribe.services.settings import get_setting
+from scribe.services.settings import bounded_float, get_setting
 
 # A budget nobody should be able to set past. Not a tuning value — a guard on
 # the worst case, so a mistyped setting cannot turn a menu into a wall of text.
@@ -270,11 +270,8 @@ def dial_for_key(key: str) -> tuple[str, str] | None:
 async def floor_for(user_id: int, name: str) -> float:
     """This install's current floor for a surface, clamped to [0, 1]."""
     s = get_surface(name)
-    try:
-        value = float(await get_setting(user_id, s.floor_key, str(s.floor_default)))
-    except (TypeError, ValueError):
-        value = s.floor_default
-    return min(1.0, max(0.0, value))
+    raw = await get_setting(user_id, s.floor_key, str(s.floor_default))
+    return bounded_float(raw, s.floor_default)
 
 
 async def budget_for(user_id: int, name: str) -> int: